To set pixels per inch, make the black line segment exactly one inch long:

Hold a ruler (in inches) up to your screen so that the zero point is aligned with the left edge of the black line segment (inside the end bar).

Click in the gray space where your ruler measures one inch, or drag the line segment to resize it.

When the black line segment is exactly one inch long, between the end bars, press OK.

When you change your screen resolution, the application resets the calibration to “(not to scale)”,
but it remembers the previous value of pixels per inch until you explicitly change it,
because it will still be valid if you return to the previous screen resolution.